This invention relates generally to the processing of a substrate utilizing a plasma in the production of integrated circuits, and specifically relates to the improvement of a plasma processing system such as one utilizing an electrostatic chuck to secure a substrate to a susceptor during processing.
Gas plasmas are widely used in a variety of integrated circuit fabrication processes, including plasma etching and plasma deposition applications, such as PECVD. Generally, plasmas are produced within a processing chamber by introducing a low-pressure process gas into the chamber and then directing electrical energy into the chamber for creating an electric field therein. The electric field creates an electron flow within the chamber which ionizes individual gas molecules by transferring kinetic energy to the molecules through individual electron-gas molecule collisions. The electrons are accelerated within the electric field, producing efficient ionization of the gas molecules. The ionized particles of the gas and the free electrons collectively form what is referred to as a gas plasma.
Gas plasmas are useful in a variety of different processes. One commonly used plasma process is a plasma etch process wherein a layer of material is removed or xe2x80x9cetchedxe2x80x9d from a surface of a substrate. In an etch process, the ionized gas particles of the plasma are generally positively charged, and the substrate is negatively biased such that the positive ionized plasma particles are attracted to the substrate surface to bombard the surface and thereby etch the substrate surface. For example, a substrate might be etched to remove an undesirable material layer or coating on the substrate before another layer is deposited. Such a pre-deposition etch process is often referred to a s etch cleaning of the substrate.
Other common plasma processes involve deposition, wherein a material layer is deposited upon the substrate. Chemical vapor deposition, or CVD, for example, generally involves the introduction of material gases into a processing chamber wherein the gases chemically interact and form a material layer or coating on the exposed substrate surface. A gas plasma can be utilized to enhance the chemical interaction. Consequently, such a CVD deposition process utilizing a plasma is referred to as plasma-enhanced CVD or PECVD. The plasma is utilized to provide energy to the process and enhance the deposition quality and/or deposition rate. Other plasma deposition processes also exist as are commonly understood by a person of ordinary skill in the art.
During plasma processing of a semiconductor substrate, it is often useful to apply an accelerating voltage to the surface of the substrate. The accelerating voltage or substrate bias is utilized to accelerate ions or other charged particles within the plasma to the substrate surface. In an etch process, the charged plasma particles are attracted to the substrate surface to actually bombard the surface and provide the etch as discussed above. In a deposition process, such as PECVD, the energy provided by such charged particle bombardment may be utilized to further enhance the deposition rate or to enhance the deposition quality, as mentioned above. Generally, biasing of the substrate in plasma enhanced etch and deposition processes is accomplished by capacitatively coupling an RF field from a susceptor or other substrate support, through the substrate, and to the exposed substrate surface which is to be etched, or which is to receive a deposited material layer. The susceptor is biased with an RF power supply and the capacitatively coupled RF field from the susceptor creates a relatively uniform DC bias potential across the substrate surface. The DC bias, in turn, affects the plasma to enhance the etch or deposition process.
Within a plasma processing system, the plasma will usually have particular non-uniformities associated therewith. For example, the plasma density is often greatest in the center of the plasma and the center of the processing chamber due to edge effects proximate the sides of the processing chamber. The non-uniformities in the plasma translate to non-uniformities and discrepancies within the etch and deposition processes in which the plasma is utilized. For example, an undesirable variation in etch rate may occur wherein the etch rate proximate the center of the substrate is greater than the etch rate proximate the outer edges of the substrate. Furthermore, within a plasma-enhanced deposition process, the deposition may be affected proximate the center of the substrate differently than at the edges of the substrate thus creating a non-uniform deposition layer and a non-uniform deposition rate radially across the wafer. It is thus an objective of the present invention to address plasma non-uniformities within plasma processing systems. It is further an objective to do so with a biased substrate.
During integrated circuit fabrication, the substrate being processed is supported within the processing chamber by a substrate support, commonly referred to as a susceptor. Oftentimes, the substrate is physically secured on the susceptor during processing, such as to improve heat transfer between the substrate and susceptor. One way of securing a substrate involves the use of an electrostatic chuck (ESC), which electrostatically attracts and secures the substrate to the susceptor. Electrostatic chucks are known in the art with one suitable design being shown in U.S. Pat. No. 5,117,121, which is incorporated herein by reference.
Generally, electrostatic chucks utilize one or more electrodes which are embedded in the susceptor. Also, the substrate might be used as an electrode. The susceptor is usually formed of a dielectric material, and an applied voltage on the electrodes causes a voltage gradient to develop within the dielectric material. The voltage gradient, in turn, affects the electrical charges on the surface of the substrate abutting the dielectric material of the susceptor such that charge differences are created between the substrate and susceptor electrodes. Thereby, the substrate is clamped to the electrostatic chuck due to attractive electrical forces between the differently charged surfaces of the susceptor electrodes and the substrate.
More specifically, with a unipolar electrostatic chuck, the voltage bias is applied to a single electrode in the susceptor and the substrate itself acts as a second electrode. In that way, the combination of the dielectric susceptor materials separating the electrode and substrate forms a parallel plate capacitor, and the attractive electrical force between the two electrodes effectively clamps the substrate to the susceptor.
Alternatively, in a bipolar electrostatic chuck, which is commonly utilized in existing processing systems, a voltage difference is applied across two or more electrodes embedded within the susceptor and spaced apart from each other. The multiple electrodes are separated by the dielectric material of the susceptor and therefore an electric field develops across the susceptor between the electrodes. When a substrate is placed on the susceptor, the electric field causes charges to accumulate on the back side of the substrate. The charges on the back side of the substrate and those on the electrodes attract one another to clamp the substrate to the susceptor. Electrostatic chucks are often utilized within plasma processing systems. Therefore, it is a further objective to address plasma non-uniformities within a processing system utilizing an electrostatic chuck.
Still further, as noted above, a substrate may be electrically biased in addition to being electrostatically clamped to a susceptor. It is therefore another objective of this invention to address the above-discussed objectives without adversely affecting the biasing of the substrate which is desirable for plasma processing.
The processing system, in accordance with the principles of the present invention, comprises a processing chamber for containing a plasma. A substrate support is mounted within the chamber for supporting a substrate proximate the plasma. A plurality of electrodes, such as first and second electrodes, are coupled to the substrate support. The electrodes are each positioned proximate the supporting surface and are electrically isolated from one another. An RF power source is coupled to each of the electrodes for biasing the electrodes with RF electrical energy. The biased electrodes are operable in conjunction with the substrate support, which is usually formed of a dielectric material, for creating a DC bias on a substrate positioned on the supporting surface of the substrate support. Generally, the electrodes are embedded within the dielectric material of the substrate support and the DC bias on the substrate is formed in accordance with known biasing principles.
In accordance with the present invention, electrically capacitive structures are used in conjunction with the electrodes to selectively vary the biasing created by the electrodes and to enhance the plasma process. In one embodiment, a first electrically capacitive structure is electrically coupled between the RF power source and at least one of the plurality of electrodes. The first electrically capacitive structure has a variable capacitance, which may be adjusted for selectively varying the DC bias created on the substrate by that electrode relative to the DC bias created on the substrate by the other electrodes of the plurality. In that way, the effect of the plasma on one portion of the substrate may be selectively varied relative to the effect of the plasma on another portion of the substrate, to achieve a desired result. The positioning of the electrodes and their selective biasing may be chosen to affect the plasma in numerous ways, such as to vary the etch or deposition rates proximate a selected area of the substrate.
In accordance with one embodiment of the present invention, the electrically capacitive structure is a variable capacitor, such as an air capacitor or a vacuum capacitor, and the capacitance of the variable capacitor may be selectively increased or decreased in accordance with the principles of the present invention. In another embodiment of the present invention, a second electrically capacitive structure, such as a second variable capacitor, is coupled to another of the electrodes so that the DC bias created by at least two of the electrodes may be selectively varied relative to each other to thereby further vary the effect of the plasma on separate portions of the substrate.
To maintain a constant power load on the RF power supply, the first and second capacitors might be operatively coupled together so that their capacitances might be varied in synchronization. In that way, the DC bias created on the separate electrodes may be selectively varied, such as by increasing the capacitance of one capacitor while decreasing the capacitance of the other capacitor, while still maintaining a generally constant power load on the RF power source coupled to the electrodes. In an embodiment of the present invention utilizing multiple capacitors, the capacitors are coupled to the RF power source in an electrically parallel orientation.
In accordance with another aspect of the present invention, the plurality of electrodes may take any number of particular shapes and positions in the susceptor necessary for selectively varying the bias on selected portions of a substrate. For example, in one embodiment of the invention, the plurality of electrodes comprises two electrodes, with one in the form of a disk for being positioned proximate the center of a substrate, while the other electrode is in the form of a ring for being positioned concentrically around the disk proximate an annular portion of the substrate. In that way, the center portion of the substrate may be biased differently from an annular portion of the substrate, such as the peripheral edges of the substrate. In another embodiment, the electrodes may take the form of multiple portions of a disk, such as two halves of a circular disk-shaped electrode.
In accordance with another aspect of the present invention, the electrodes and variable capacitors may be incorporated into a processing system in combination with an electrostatic clamping device. To that end, the electrodes may also be coupled to a DC power source which is operable for creating a DC potential difference between at least two of the electrodes of the plurality to electrostatically clamp a substrate to the supporting surface of the substrate support. To that end, the electrodes are used for both electrostatically clamping the substrate and for selectively affecting the plasma proximate the substrate in accordance with the principles of the present invention. Generally, due to the physical nature of the electrostatic clamping force, the surface of the substrate, which is biased by the RF-created DC bias to affect the plasma, is not affected by the electrostatic clamping force created by the DC power source.
These features and benefits of the invention, and other features and benefits are set forth in greater detail in the Detailed Description hereinbelow made in reference to the drawing figures.